Recently, as measures against global warming, electric power generation that uses natural energy (co-called renewable energy), for example, photovoltaic power generation or wind power generation, has been actively conducted globally. The output of such power generation depends significantly on natural conditions such as the weather. Therefore, when the ratio of electric power derived from natural energy to the total generated output increases, the occurrence of a problem in terms of operation of the electric power system, for example, a problem in that it becomes difficult to maintain the frequency and the voltage, is expected. One measure against this problem is to install a storage battery having a large capacity so as to achieve smoothing of an output fluctuation, accumulation of surplus power, load leveling, and the like.
An example of such a storage battery having a large capacity is a redox flow battery (hereinafter, also referred to as “RF battery”). A redox flow battery is a secondary battery that performs charging and discharging by supplying a positive electrode electrolyte and a negative electrode electrolyte to a battery cell that includes a positive electrode, a negative electrode, and a separating membrane interposed between the two electrodes. Redox flow battery electrolytes used in such a redox flow battery usually utilize, as active materials, metal ions whose valences change as a result of oxidation-reduction. Examples of the redox flow battery include an iron (Fe2+/Fe3+)-chromium (Cr3+/Cr2+)-based redox flow battery that uses an iron (Fe) ion as a positive electrode active material and uses a chromium (Cr) ion as a negative electrode active material and a vanadium (V2+/V3−—V4+/V5+)-based redox flow battery that uses V ions as active materials of two poles.